Prior attempts to reduce tar and harmful carcinogenic nitrosamines primarily have included the use of filters in smoking tobacco. In addition, attempts have been made to use additives to block the effects of harmful carcinogens in tobacco. These efforts have failed to reduce the oncologic morbidity associated with tobacco use. It is known that fresh-cut, green tobacco has virtually no nitrosamine carcinogens. See, e.g., Wiernik et al, "Effect of Air-Curing on the Chemical Composition of Tobacco," Recent Advances in Tobacco Science, Vol. 21, pp. 39 et seq., Symposium Proceedings 49th Meeting Tobacco Chemists' Research Conference, Sep. 24-27, 1995, Lexington, Ky. (hereinafter "Wiernik et al."). On the other hand, cured tobacco products obtained according to conventional methods are known to contain a number of nitrosamines, including the harmful carcinogens N'-nitrosonornicotine (NNN) and 4-(N-nitrosomethylamino)-1-(3-pyridyl)-1-butanone (NNK). It is widely accepted that such nitrosamines are formed post-harvest, during the conventional curing process, as described further herein. Unfortunately, fresh-cut green tobacco is unsuitable for smoking or other consumption.
It is believed that tobacco-specific nitrosamines (TSNAs) are formed primarily during the curing process. While not wishing to be bound by theory, it is believed that the amount of tobacco-specific nitrosamine (TSNA) in cured tobacco leaf is dependent on the accumulation of nitrites, which accumulate during the death of the plant cell and are formed during curing by the reduction of nitrates under conditions approaching an anaerobic (oxygen deficient) environment. It is believed that the reduction of nitrates to nitrites occur by the action of the micro flora on the surface of the leaf under anaerobic conditions, and it is also believed that this reduction is particularly pronounced under certain conditions (e.g., humid conditions). Furthermore, during the curing process, the tobacco leaf emits carbon dioxide, which can further dilute oxygen levels in the environment.
Once the nitrites are formed, these compounds are believed to combine with various tobacco alkaloids, including pyridine-containing compounds, to form carcinogenic nitrosamines.
In 1993 and 1994, Burton et al at the University of Kentucky carried out certain experiments regarding TSNA, as reported in the Abstract, "Reduction of Nitrite-Nitrogen and Tobacco N'-Specific Nitrosamines In Air-Cured Tobacco By Elevating Drying Temperatures," Agronomy & Phytopathology Joint Meeting, CORESTA, Oxford 1995. Burton et al reported that drying harvested tobacco leaves for 24 hours at 71.degree. C., at various stages of air curing, including end of yellowing (EOY), EOY+3, EOY+5, etc. resulted in some reduction of nitrosamine levels. Reference is also made to freeze drying and microwaving of certain samples, without detail or results. It has been confirmed that in the actual work underlying this Abstract, carried out by Burton et al at the University of Kentucky, the microwave work was considered unsuccessful. Certain aspects of Burton et al's 1993-94 study are reported in Wiernik et al, supra, at pages 54-57, under the heading "Modified Air-Curing." The Wiernik et al article postulates that subjecting tobacco leaf samples, taken at various stages of air-curing, to quick-drying at 70.degree. C. for 24 hours, would remove excess water and reduce the growth of microorganisms; hence, nitrite and tobacco-specific nitrosamine (TSNA) accumulation would be avoided. In Table II at page 56, Wiernik et al includes some of Burton et al's summary data on lamina and midrib nitrite and TSNA contents in the KY160 and KY171 samples. Data from the freeze-drying and the quick-drying tests are included. The article contains the following conclusion:
It can be concluded from this study that it may be possible to reduce nitrite levels and accumulation of TSNA in lamina and midrib by applying heat (70.degree. C.) to dark tobacco after loss of cell integrity in the leaf. Drying the tobacco leaf quickly at this stage of curing reduces the microbial activity that occurs during slow curing at ambient temperature. It must be added, however, that such a treatment lowers the quality of the tobacco leaf.
Id. at page 56. The Wiernik et al article also discusses traditional curing of Skroniowski tobacco in Poland as an example of a 2-step curing procedure. The article states that the tobacco is first air-cured and, when the lamina is yellow or brownish, the tobacco is heated to 65.degree. C. for two days in order to cure the stem. An analysis of tobacco produced in this manner showed that both the tobacco-specific nitrosamine (TSNA) and the nitrite contents were low, i.e., in the range of 0.6-2.1 micrograms per gram and less than 10 micrograms per gram, respectively. Wiernik et al theorized that these results were explainable due to the rapid heating which does not allow further bacterial growth. Wiernik et al also noted that tobacco-specific nitrosamine (TSNA) and nitrite contents of 0.2 microgram per gram and less than 15 micrograms per gram, respectively, were obtained for tobacco subjected to air-curing in Poland.
In practice, tobacco leaves are generally cured according to one of three methods. First, in some countries, such as China, a variation of the flue curing process (described below) is still being used on a commercial scale to cure tobacco leaves. Specifically, this variation of the flue curing process features the use of a heat exchanger and involves the burning of fuel and the passing of heated air through flue pipes in a curing barn. Accordingly, in this older version of the curing process, primarily radiant heat emanating from the flue pipes is used to cure the tobacco leaves. While a relatively low flow of air does pass through the curing barn, this process utilizes primarily radiant heat emanating from the flue pipes to cure the tobacco leaves within the barn. In addition, this process does not appreciate, and does not provide for, controlling the conditions within the barn to achieve prevention or reduction of TSNAs. This technique has been largely replaced in the United States by a different flue-curing process.
For more than twenty years, the heat exchanger method described above has been supplanted in the U.S. with a more economical version which features the use of a propane burner. This second method is the so-called "flue curing" method. This process involves placing the tobacco leaves in a barn and subjecting the leaves to curing with the application of convective heat using a hot gaseous stream that includes combustion exhaust gases. When convective heat is used to dry the tobacco leaves, the combustion exhaust gases (including carbon monoxide, carbon dioxide, and water) are passed directly through the tobacco. In processes where convective heat is used for curing, no attempt is made to separate the heat from the combustion exhaust gases (i.e., to prevent an anaerobic condition) or to control the ambient conditions to reduce or suppress the formation of TSNAs.
The third method is known as "air curing." This process involves placing the tobacco leaves in a barn and subjecting the leaves to air curing without controlling the ambient conditions (e.g., air flow through the barn, temperature, humidity, and the like) and without the application of any heat.
U.S. Pat. No. 2,758,603 to Heljo discloses a process for treating tobacco with relatively low moisture levels (i.e., already cured tobacco) with radio frequency energy to accelerate the aging process. Although the patent states that the tobacco being treated is "green" tobacco, it is clear that the patent is using the term "green" in a non-conventional sense because the tobacco being treated therein is already cured (i.e., the tobacco is already dried). This is clearly evident from the disclosed moisture levels for the tobacco being treated in the Heljo patent. In fact, Heljo rehydrates the fully cured tobacco prior to the radio frequency treatment. By contrast, in the present invention, the term "green tobacco" refers to freshly harvested tobacco, which contains relatively high levels of moisture.
Additionally, the use of microwave energy to dry agricultural products has been proposed. For example, use of microwave energy to cure tobacco is disclosed in U.S. Pat. No. 430,806 to Hopkins. Further, U.S. Pat. No. 4,898,189 to Wochnowski teaches the use of microwaves to treat green tobacco in order to control moisture content in preparation for storage or shipping. In U.S. Pat. No. 3,699,976, microwave energy is described to kill insect infestation of tobacco. Still further, techniques using impregnation of tobacco with inert organic liquids (U.S. Pat. No. 4,821,747) for the purposes of extracting expanded organic materials by a sluicing means have been disclosed wherein the mixture was exposed to microwave energy. In another embodiment, microwave energy is disclosed as the drying mechanism of extruded tobacco-containing material (U.S. Pat. No. 4,874,000). In U.S. Pat. No. 3,773,055, Sturgis discloses the use of microwave to dry and expand cigarettes made with wet tobacco.
Using a novel breakthrough curing technology, U.S. Pat. No. 5,803,081 to Williams discloses a method of reducing the nitrosamine levels or preventing the formation of nitrosamines in a harvested tobacco plant using microwave energy.
In copending U.S. patent application Ser. No. 08/879,905, filed Jun. 20, 1997, a process for reducing the amount of or preventing the formation of nitrosamines in harvested tobacco plant is disclosed, wherein the process comprises subjecting at least a portion of the plant to microwave radiation, while the portion is uncured and in a state susceptible to having the amount of nitrosamines reduced or formation of nitrosamines arrested, for a time sufficient to reduce the amount of, or substantially prevent formation of, at least one nitrosamine.
Further, copending U.S. patent application Ser. No. 08/998,043, filed Dec. 23, 1997, discloses that microwave and other types of radiation are useful for treating tobacco to reduce the amount of, or prevent the formation of, nitrosamines in tobacco.